Reports: DNI849633-DNI8: Deposition on the Far End: Shale Facies Architecture as an Archive for High-frequency Cyclicity

Sven O. Egenhoff, PhD, Colorado State University

Aims of project and progress

The aim of this project is to (1) identify different facies in the Alum and Tøyen shales, both Early Paleozoic black shale/mudstone units in Scandinavia, (2) use the identified shale/mudstone facies and their relationship to over- and underlying carbonate rocks to establish a depositional model for these fine-grained siliciclastic rocks, and (3) document hitherto non-described small-scale cyclicity such as Milankovitch and sub-Milankovitch rhythms in mudstone successions where present. The investigations are based on four cores (Andrarum, Tomten, Lerhamn and Krapperup) all housed at the Geology Department of Lund University, Sweden, and several detailed documentations of the outcrops in Hunneberg and Kakeled (Västergötland, southern central Sweden) and Slemmestad (south of Oslo, Norway).

The Alum Shale analysis used time-equivalent sections in two outcrops (Kakeled and Mount Hunneberg) and the Tomten core to establish a depositional model for the Peltura scarabeoides trilobite Biozone (Upper Cambrian) based on continuous thin section coverage, and to compare the internal facies architecture of these three sites with an emphasis on detecting small-scale cycles. In a subsequent study detailed thin section analysis of the Andrarum core rocks (Middle to Upper Cambrian) investigated sediment deposition on the deep shelf during a proposed Cambrian Ocean Anoxic Event.

The data for this project was collected during two trips to Sweden and Norway in August of 2010 and 2011. Cores were measured at the Geology Department of Lund University, Sweden, while outcrop documentation was undertaken in the field at Mount Hunneberg, Kakeled (both Västergötland), and Slemmestad (Norway). The PI and the graduate student presented results from this project on the AAPG ACE meeting in Houston in April 2011, and on the annual GSA national meeting in Minneapolis, Minnesota, in October of 2011. One manuscript based on data from this project has been submitted to GFF (Egenhoff and Maletz, submitted), and two other manuscripts are in progress. One of them is an invited manuscript for a GSA Special Paper on mudstones as an outcome of the presentation during the Minneapolis GSA meeting. The graduate student thesis is close to completion and will be defended in October or November this year. The PI is continuing to cooperate with professors Mikael Calner and Per Ahlberg at the Geology Department of Lund University. An ongoing cooperation with Dr. Jörg Maletz, Berlin, Germany, will result in another manuscript as an outcome of this project describing preservation and deposition of graptolites. Data from this project have also lead to joint work with Dr. Oliver Lehnert, Erlangen University, Germany which resulted in a first overview manuscript on Scandinavian geology (Lehnert et al. 2012).

Results

While the research is in part still ongoing the results at present are as follows:

(a) The Alum Shale in the Peltura scarabeoides trilobite Biozone shows that both the carbonate as well as the siliciclastic mudstone environments were subdivided into several distinct facies belts during the Late Cambrian. Three carbonate and three shale facies belts are recognized with a trend of decreasing energy downramp. The succession reflects bed-load transport even in the shale portion of this ramp as indicated by lags and mudstone ripples; however, sediments from suspension deposition are not easily recognizable because of ubiquitous and intense bioturbation in every facies belt.

(b) Not only the Upper Cambrian sediments, but also the Middle Cambrian deposits are characterized by abundant synsedimentary burrowing activity. This indicates that the Cambrian deep shelf was at least dysoxic and not anoxic or euxinic as suggested by Gill et al. (2011). While this does not preclude the presence of an anoxic event in the Furongian (Late Cambrian) it shows that the deep shelf was highly populated with benthic organisms during the critical time interval at the base of the Upper Cambrian.

(c) Unlike the Alum Shale, the Tøyen Shale shows an overall more proximal depositional setting with abundant influx from a nearshore carbonate factory. The depositional model is most likely more similar to modern deep shelf and basinal portions of ramp settings, and processes observed in Tøyen Shale sediments should also characterize other black shale successions throughout the rock record. Nevertheless, both Alum and Tøyen Shales lack a distal chert-rich facies such as e.g. the Bakken or Woodford systems. This is related to radiolarian evolution with these planktic organisms becoming frequent in deep waters only since the Silurian.

(d) Decimeter- and meter thick cycles are abundant in both the Alum and the Tøyen Shale successions. However, only the distalmost facies belt of the Alum Shale shows millimeter-scale mudstone cycles that generally coarsen upward. These cycles are thought to represent sea level fluctuations related climate changes, most likely in the sub-Milankovitch band.

(e) Mount Hunneberg in south-central Sweden exhibits a Lower Ordovician succession dominated by siliciclastic mudstones with few carbonate intercalations. Above an unconformity with mudcracks reflecting initial exposure, the Tremadocian succession shows deposition of two centimeter-thick shale intervals with graptolites recording initial flooding and subsequent regression. Overlying glauconite packstones represent transgressive deposits with the Late Tremadocian Bjørkåsholmen Formation recording a distinct lowstand. The overlying Tøyen Shale has a transgressive character reflected in overall fining-upward. The mudcracks, their association with shallow-marine carbonates and the sedimentary structures in the Ordovician shales show that the fine-grained and in part TOC-rich siliciclastic rocks could not have been deposited in a deep-marine setting, but instead were laid down in a relatively shallow water shelf environment, mostly above or near storm wave base (Egenhoff and Maletz, submitted) .

References

Egenhoff, S., and Maletz, J.: The sediments of the Floian GSSP - depositional history of the Ordovician succession at Mount Hunneberg, Västergötland, Sweden. GFF, submitted.